The present invention is concerned with hard disk drives, and is more particularly concerned with processing a readback signal provided by a hard disk drive.
Hard disk drives are well known components of computer systems. A typical hard disk drive includes one or more data storage disks for storing data in magnetic form and a transducer used to write and read magnetic data respectively to and from the data storage disk. The data storage disk is mounted on a hub of a spindle motor which rotates the disk at speeds typically on the order of several thousand revolutions-per-minute. Digital frequency modulation information is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disk. An actuator assembly adjustably positions the transducer above the data storage disk as the disk rotates. It is known to use an MR (magnetoresistive) element as a read transducer. The MR read transducer typically uses a constant bias current passing through the MR element. The magnetic information stored on the disk will modulate the MR resistance and thus changes the readback signal voltage.
A problem encountered with MR read transducers in hard disk drives is thermal modulation of the readback signal voltage produced by the MR read transducer. The thermal modulation of the readback signal voltage may interfere with recovery of data from the readback signal.
Thermal modulation may come about in a number of ways. For example, the MR head of an MR read transducer may come into contact with a burr or protrusion on the data storage disk. Such contact causes rapid frictional heating of the MR head. Because of the positive thermal coefficient of resistance of the MR head, the resistance of the MR head is rapidly increased. With the constant bias current applied to the MR head, the increase in resistance leads to a large positive excursion in the readback signal voltage. An event of this type is referred to as a xe2x80x9cthermal asperityxe2x80x9d.
FIG. 1(a) is a graphical representation of a hard disk drive readback signal modulated at location 10 by a thermal asperity. FIG. 1(b) is a graphical illustration of the thermal component of the readback signal.
The readback signal from the MR head can also be thermally modulated by non-contact heating or cooling from the data storage disk. The thermal modulation is caused by variations in heat transfer from the MR head to the disk. Because of the constant bias current supplied to the MR head, it is heated above ambient temperature and tends to be cooled by the proximity of the data storage disk. When the MR head travels over a pit in the data storage disk, the greater distance separating the MR head from the data storage disk results in a decrease in the cooling effect of the data storage disk upon the MR head, so that the temperature of the MR head increases, leading to increasing thermal modulation of the readback signal produced by the MR head. Also, when the MR head travels over a bump on the data storage disk (without contact between the MR head and the data storage disk) the closer proximity of the MR head to the surface of the data storage disk at the locus of the bump leads to increased cooling of the MR head by the data storage disk. This lowers the temperature of the MR head, due to greater heat transfer leading to a decreased thermal modulation of the readback signal produced by the MR head. These types of thermal modulation of the readback signal are sometimes referred to as xe2x80x9cbaseline signal wanderxe2x80x9d.
U.S. Pat. No. 5,751,510, which has the same inventors and the same assignee as the present invention, discloses a circuit that is adapted to remove a thermal modulation component from the readback signal produced by an MR head of a hard disk drive. The ""510 patent is incorporated herein by reference in its entirety. According to the circuit shown in FIG. 9 of the ""510 patent, the readback signal from the MR head is sampled and digitized and then a digital filter having a lowpass characteristic extracts the thermal component from the digitized readback signal. The thermal component is then subtracted from the readback signal (after the readback signal has been delayed to compensate for the delay in the filter) to produce a restored magnetic readback signal that is substantially free of thermal modulation.
While the ""510 patent represents an advance in the art, the present inventors have found that it would be desirable to remove thermal modulation from a hard disk drive readback signal provided by a MR head in a manner that copes more satisfactorily with the nonlinear characteristics of the readback signal as it reflects actual data stored on the data storage disk.
According to an aspect of the invention, a method for processing a signal obtained from a storage medium using a magnetoresistive (MR) element is provided. The MR signal may be either a voltage or a current signal. The method includes reading the signal from the storage medium using the MR element and sampling and digitizing the signal to obtain a digital readback signal. The method further includes estimating an envelope of the digital readback signal by applying a nonlinear digital process to the digital readback signal to obtain an upper envelope sequence and a lower envelope sequence. The method further includes deriving an estimated thermal component of the digital readback signal from the upper envelope sequence and the lower envelope sequence, and subtracting the estimated thermal component from the digital readback signal to generate a restored digital readback signal.
The estimation of the thermal component of the digital readback signal may include examining the digital readback signal within each of a sequence of timing windows. Further, the method may include, for each of the timing windows, finding a second highest value of the digital readback signal in the respective window to indicate the upper envelope sequence, and finding the second lowest value of the digital readback signal in the respective window to indicate the lower envelope sequence. The step of deriving the estimated thermal component of the digital readback signal may include obtaining an arithmetic mean of the upper and lower envelope sequences.
Computer program products may be provided in accordance with these and other aspects of the invention. Each inventive program product may be carried by a medium readable by a computer (e.g., a carrier wave signal, a floppy disk, a hard drive, a random access memory, etc.).
With the present invention, removal of a thermal modulation component from the readback signal produced by an MR head from a hard disk drive is performed satisfactorily, notwithstanding changes in frequency that are characteristic of the frequency modulated data stored in actual practical use of the hard disk drive.